2 Answers
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There is no international ferry operating on lake Victoria, scheduled or otherwise. There is a scheduled ferry between Mwanza and Bukoba, both in Tanzania.

It is possible, however, to travel from, specifically, Kampala to Mwanza, by boat. But, this basically requires going to the port and asking around for a berth on one of the cargo ships.
I've not heard of anyone managing this between Kampala and Kisumu, but if you can find a cargo ship doing that route, you're likely able to find yourself a bed too.

In case you're wondering, I currently live in Kampala and have looked into crossing lake Victoria on several occasions myself.

I have no reliable information, but I'll post this anyway as an answer until someone finds reliable sources.

The traditional Lake Victoria ferries were discontinued in the mid-2000s after a series of accidents. The Ugandan government announced that it would bring in a replacement service but nothing came of that. Earthwise operates one ferry out of Uganda with two more to come, but they do not serve Kisumu.

It seems that there is no current ferry service between Kisumu and Entebbe.